Before the 20th century, explained Smith, little was expected of military medicine; disease killed far more troops than combat did. “But with the rise of germ theory, and the development of aseptic surgery as the world moved into World War I,” said Smith, “there was a fundamental mindset change. Medicine seemed to be making so much progress that perhaps it could change the outcome of war for some people.” The induction physical performed by military physicians was far more thorough and informative than those performed, for example, on Civil War inductees.

“But,” Smith said, “we still didn’t know what we didn’t know.” World War I inductees were mostly young, healthy men, tested for obvious problems: hernias, heart defects, dental problems, and vision and hearing impairments. They were not, despite the generally widespread exposure to Mycobacterium tuberculosis among 19th century Americans, tested for the latent form of tuberculosis (TB). That became a problem when young veterans began returning home from Europe, where the Germans and Allies both made use of poison gas weapons, and complained of respiratory trouble. Many of these veterans came down with active TB, and made the argument that their exposure to chemical weapons had been the factor that led to their pulmonary problems. Without any evidence to argue otherwise, the Veterans Bureau compensated large numbers of World War I veterans and treated them in hospitals.

The government made notable attempts to learn more about what it didn’t know during and after World War II. Because the damaging effects of radiation were well-known by the time the United States dropped atomic bombs on Hiroshima and Nagasaki in 1945, the government commissioned studies among veterans who were part of the ensuing occupations of the two cities, were prisoners of war in the area, or participated in the above-ground nuclear tests conducted between 1945 and 1962. Evaluations of illness and mortality among “atomic veterans” continued for decades, and in accordance with these findings, Congress authorized the VA to establish disability ratings and benefit payments to veterans who had been exposed, to any degree, to ionizing radiation. In 1986, it also directed the VA to create an Ionizing Radiation Registry, allowing atomic veterans to receive free health examinations and to receive information about the potential long-term consequences of their exposures. As of May 2015, more than 25,000 veterans have signed up to participate in the Ionizing Radiation Registry program.

One of the most studied cases of deployment-related exposure occurred during the Vietnam War, when U.S. military forces sprayed more than 19 million gallons of herbicides over the region’s tropical foliage to expose concealed opposition forces, destroy crops, and clear perimeters around U.S. bases. As anecdotal and clinical evidence emerged that these defoliants – particularly a contaminant in the herbicide known as Agent Orange – could cause a variety of health problems, a familiar series of events unfolded: Both the White House and Congress commissioned studies; Congress passed a series of laws focusing on health care and compensation for veterans exposed to Agent Orange; and the VA launched both a registry and compensation program for veterans who had one of the disorders for which there was a scientific association with herbicide exposure.

The first Gulf War of 1990-1991 presented a new challenge to the federal government’s approach to handling military exposures: a chronic multi-symptom disorder, affecting veterans and civilian workers returning from the Persian Gulf, that became known as Gulf War syndrome. IOM studies of returning veterans have revealed much higher rates of chronic multi-symptom illness among Gulf War veterans than among the general population: About 250,000 of the 700,000 U.S. personnel deployed to the region, the report said, suffered from persistent, unexplained symptoms including fatigue, muscle and joint pain, rashes, and cognitive problems. Nine IOM reports filed since 1998 have catalogued a unique combination of hazards never before experienced during wartime, including medications given to protect troops from nerve agents, munitions containing depleted uranium, organophosphate pesticides, vaccines for anthrax and botulinum, and persistent smoke from oil well fires.

Despite many circumstantial differences, the government’s response to exposures in World War II, Vietnam, and the Gulf War shared one key similarity, according to Smith: Despite an obvious effort to anticipate, prevent, and document deployment-related exposures, the military and the VA still didn’t know much about what they didn’t know.

Some of these factors have since been ruled out by investigators as causes for these symptoms, but because there is still no identifiable cause for what’s known as Gulf War syndrome, its history is riddled with controversy. The short version of the outcome is that the VA has both a Gulf War Registry program, and has also extended its disability compensation to Gulf War veterans with “medically unexplained illnesses” and certain infectious diseases associated with Southwest Asia.